1. Technical Field
The present invention relates generally to simultaneous broadcast, or Simulcast, networks, and more specifically to a method of synchronization on the uplink channel of such a network.
The invention should find application, in particular, in Professional Mobile Radio (PMR) systems, in which a Simulcast network functionality is currently provided.
2. Related Art
Generally, the term “network” is used to mean the fixed infrastructure of the system. This fixed infrastructure comprises, connected via a backbone, on the one hand the network elements (hereinafter called the network subsystem) which manage the functionalities of the system, and on the other hand the radio transmission and/or reception equipment (hereinafter called the radio subsystem) which handle the air interface with the mobile terminals and which are distributed in such a way as to cover individual geographic areas called cells. When radio transmission means and radio reception means are co-located within one and the same element, this element forms what is called a base station.
The term “Simulcast network” is used to mean all of the elements of the radio subsystem involved in a Simulcast configuration, under the control of the network subsystem. Generally, the Simulcast network comprises the entire radio subsystem, but this is not mandatory.
Among the resources of the network subsystem that are involved in managing the Simulcast network, there is what is called a selection unit or function. The role of this is to choose one or more of the base stations according to a criterion associated with the quality of the radio link on each uplink (link originating at the mobile terminal and terminating at the base station). In practice, a number of base stations can receive the radio signal transmitted by the mobile terminal in conditions enabling them to process it to extract the useful information contained in it, with varying quantities of errors. When the radio signal is organized in frames, these base stations redundantly provide the network subsystem with useful information frames. It is therefore desirable to elect, in other words select (according to quality information associated with each frame) just one stream of frames originating from a predefined base station and disregard the other streams of frames, or, as a variant, to combine several or all of the streams of frames received (using similarity information associated with each frame). The entity performing this operation is the aforementioned selection function. This can be centralized, in other words executed within a predefined element, or distributed over a number of elements.
As is diagrammatically illustrated in FIG. 1, a Simulcast configuration enables the radio coverage to be extended by creating at least one macrocell MC made up of several individual radio cells C1 to C6, all using the same frequency channel. Thus, a mobile terminal MS located in any of the cells C1 to C6 is able to receive and/or send radio signals from, respectively to, the network. In other words, the mobile terminal MS can handle the current call in the Simulcast network without needing any resource management taking account of its position.
The cells C1 to C6 forming the macrocell MC are normally adjacent. There are therefore overlaps between the different cells that make up the macrocell. The mobile terminal can thus roam from one cell to another without the call being interrupted, and in a manner transparent to it. For the mobile terminal, the macrocell is therefore seen as a single cell.
The present invention is not concerned with the call management policies in a Simulcast network. Consequently, in the interests of simplicity, we will consider here the case of a single mobile terminal involved in a call at a defined moment. The only concerns here will be the techniques of synchronization, both on the downlink channel (in other words from the network to the mobile terminal) and on the uplink channel (in other words from the mobile terminal to the network) of a Simulcast network.
The invention relates in practice to resolving the problem of synchronization on the uplink channel of a Simulcast network.
On each base station, synchronization on the uplink channel can be achieved according to various known principles.
Conventionally, the mobile terminal sends a synchronization sequence, known to the base stations, and having properties enabling such synchronization. For example, the synchronization sequence can be a pseudo-random sequence, with a self-correlation property. Normally, the synchronization sequence is sent by the mobile terminal at the start of the call in the case of a full-duplex mode call, or at the start of each alternate turn in the case of a half-duplex mode call.
Thus, at the start of each call (or at the start of each alternate turn), at least one base station can detect this synchronization sequence and synchronize on the uplink channel, provided that the signal is received with a sufficiently strong power level and with an adequate signal-to-noise ratio. It will be noted that in this case, if the call (or the alternate turn) lasts a long time and the mobile terminal is roaming, the radiofrequency propagation conditions may be altered during the call (or alternate turn) and a new base station may receive a signal originating from the mobile terminal with a sufficiently strong power level to enable it to demodulate the signal correctly.
Similarly, the base station which is synchronized at the start of the call (or alternate turn) may see the power level of the received signal decrease and then become incapable of correctly demodulating that signal. This problem is particularly troublesome for full-duplex mode calls which can last several minutes and for which there is no alternate turn.
A conventional solution to this problem is to have the mobile terminal transmit a synchronization sequence at regular intervals, at times known in principle by the base stations. This method is effective and simple. However, it reduces the available bandwidth on the uplink channel, and therefore, potentially, the audio quality of the speech information to be transmitted.
As a variant, it is also conventional to use, as the synchronization sequence on the uplink channel, a fixed sequence typically placed at the start of each radio burst. Such a sequence is often provided in mobile radio communication systems. It is normally called a training sequence because it can also be used to estimate the radio channel or to initialize its estimation. This sequence is often shorter than the synchronization sequence described in the preceding paragraph. Furthermore, it does not necessarily offer very good self-correlation properties. It is therefore often difficult to use it to synchronize the base stations in a Simulcast network. It may be much too short to enable synchronization with an adequate probability of success.
Furthermore, at the first synchronization level hitherto described, it is often necessary to associate a fine synchronization procedure, in other words, with a precision greater than the duration of a symbol time. Such a procedure can, in certain cases, use the specific characteristics of the signal (called the modulation signature). It should be understood from this that this procedure is not based on signalling information inserted for that purpose in the radio signal. That is possible, for example, with GMSK modulation. The invention seeks to resolve, in a Simulcast network, the problem of synchronization on the uplink channel, without increasing the complexity of the mobile terminal or of the base stations (or at least the network receivers), without reducing the bandwidth of the radio signal, by enabling the base stations (or network receivers) to switch to receive processing mode as quickly as possible after receiving a radio signal originating from the mobile terminal with a sufficiently high power level, and by offering performance as good as the conventional solution by sending synchronization sequences at the start of the call (or at the start of the alternate turn) then sending synchronization sequences at regular intervals.